Like the bench press is the king of chest exercises, the barbell row is ruler of its domain for the lats and traps. With most other lat exercises, you don't directly fight gravity - a pulley eases the struggle on cable exercises. And with the barbell row, you can really load up on the weight, making it great for upper back development. From a true strength standpoint, this exercise is a staple of most elite athlete programs. And when it comes to sheer size, no lifter should leave this lift out of his routine.
Take a wide-grip palms down approach while stabilizing your feet at shoulder width and bending your knees about 15 degrees. Your bent-over position should not be parallel with the ground but at about a 60 degree trunk angle. If you start by retracting (pulling in) your scapulae you will further isolate your lats, but you will need slightly lighter weight. Some prefer the rounded upper back start and retract during the lift to make the traps work in conjunction with the lats. In either case, stick your chest out, flatten your lower back and pull the weight hard into your upper abdomen while sending your elbows backward. Return the weight under control and pause to avoid momentum from taking over.
While the barbell version is superior for overall development, using cable and dumbbell variations works well too. For a more refined barbell version that uses a little more biceps, use a reverse grip (palms up) just outside your hips on the same bent over row movement. Your pull will be more to your lower abdomen and shift the emphasis to your lower lats, with some help from your bi's. Some gyms also have a modified barbell with neutral handles.